AnkiDroid is a flashcards (http://en.wikipedia.org/wiki/Flashcard) application for Android.

It is free (and open source (http://en.wikipedia.org/wiki/FOSS)), you can download it from the Android Market (https://market.android.com/details?id=com.ichi2.anki) or as an APK (http://code.google.com/p/ankidroid/downloads/list).